Archetypal Soft-Mode Driven Antipolar Phase Transition in Francisite Cu3Bi(SeO3)2O2Cl

Sep 11, 2019, 4:05 PM
Student Communication Afternoon Session 1


Cosme Milesi-Brault (Luxembourg Institute of Science and Technology)


Antiferroelectricity can be seen as being a property similar to antiferromagnetism with electric dipoles instead of spins. It is characterized by a phase transition between a high- and a low-symmetry phase where antiparallel dipoles emerge [1]. In analogy with soft-mode driven ferroelectric transitions (e.g. in PbTiO3), it is then possible to think of an ideal antiferroelectric phase transition driven by an “anti-polar soft-mode”, which is a soft phonon mode related to antiparallel ionic displacements [2]. However, such a phase transition has not been observed yet; instead, classical antiferroelectric transitions are usually of the order-disorder type.

In this study, we show that francisite (Cu3Bi(SeO3)2O2Cl) undergoes such an anti-polar soft-mode driven phase transition. Francisite is an orthorhombic crystal that has a phase transition from space group Pmmn to Pcmn at 115 K [3]. This phase transition induces a doubling of the unit cell along the c axis, which folds the zone-boundary Z point (0,0,1/2) of the high-symmetry phase onto the Γ point in the antipolar phase [4]. We measured the low-frequency phonon modes in both phases using a combination of Raman spectroscopy, Inelastic X-Ray Scattering (IXS) and Thermal Diffuse Scattering (TDS). IXS and TDS measurements have been performed at the ID 28 beamline of ESRF [5].

Raman spectra across the phase transition are shown on Fig. 1. and show a clear soft phonon mode visible only in the low-temperature phase. The soft-mode above Tc on the other hand is seen in the IXS spectra and in the TDS intensity. Fig. 2 shows the combination of experimental data with the soft-mode energy squared as a function of temperature. It displays a typical soft-mode behaviour in the vicinity of Tc, with deviations originating from mode coupling with other low lying phonon modes. The slopes of E2 vs. T indicate that the transition is close to tricritical.

Fig. 1 is the enclosed "Raman_cascade.jpg" file and Fig 2. is "Soft-mode_evolution.jpg"

Topic 2. Multiferroics and ferroelectrics

Primary authors

Cosme Milesi-Brault (Luxembourg Institute of Science and Technology) Dr Evan CONSTABLE Dr Hugo Aramberri (Luxembourg Institute of Science and Technology) Dr Virginie Simonet (Institut Néel) Sophie DE BRION (Insititut Néel) Dr Helmuth Berger (École Polytechnique Fédérale de Lausanne) Dr Luigi Paolasini (ESRF) Dr Alexei Bosak (ESRF) Jorge Iniguez (Luxembourg Institute of Science and Technology) Dr Mael Guennou (Physics and Material Science Research Unit, University of Luxembourg)

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